1. Field of the Invention
The present invention is directed to a method for the production of light conductor structures which have electrodes arranged between the light conductors.
2. Prior Art
Light conducting structures, which have a pair of light conductors on a substrate with an electrode disposed therebetween are known. Such structures are used as electrically controllable directional couplers in optical communication technology and act as on/off switches or cross-over switches. In addition, such structures are used as electrical-optical modulators.
A common feature of these structures is that the two light conductors possess a zone in which they are very closely adjacent to one another. Electrodes are arranged between the light conductors and beside each of the light conductors. In this zone, a typical value for the spacing between a pair of light conductors is 3 .mu.m. This means that the electrodes must be precisely aligned in their position and that permissible tolerances in the location of the electrodes are less than 1 .mu.m.
German Offenlegungsschrift No. 2,526,117 describes a method for producing this type of light conductor structure. As disclosed in this reference, a metal layer is applied to a substrate consisting of a ferroelectric material. This metal layer is removed in the regions where the light conductors are to be formed so that the surface of the substrate is exposed at these regions or areas. Thereafter, a diffusion material is applied to the metal layer and to the exposed zones or areas of the surface of the substrate. During a high temperature diffusion process, this diffusion material diffuses into the exposed zones of the substrate to increase the index of refraction of the substrate in these zones so that these zones having the increased index of refraction act as light conductors or optical waveguides. In the region of the two waveguides, opposing, remanent electric polarizations are now produced in the substrate and the metal located between the light conductors is then removed from the substrate. The remaining metallizations which are present on the surface of the substrate serve as the electrodes.
Fundamentally, this production or method could also be used for the production of electrodes located between two light conductors. As a variation of the process described in the above-mentioned German reference, the metallization located between the light conductors would then have to remain on the substrate. This metallization would then serve as an electrode, and as a result of the production process this electrode would be automatically positioned between the light conductors with a high or extreme accuracy.
Since, however, the metallizations are exposed to high temperatures during the diffusion process for the production of the light conductors, it is unavoidable that a portion of the metallizations will also diffuse into the substrate. However, this condition is extremely undesirable and would thereby reduce the quality of the light conductors. In particular, a danger exists that a light conducting layer will form beneath the metallizations.
In addition, the danger exists that the metallizations will become oxidized and thus might become electrically non-conductive. It is also possible that without special adhesives, the metallizations will not adhere sufficiently firmly upon the substrate so that at the high temperatures prevailing during the diffusion process, the metallizations can contract to form small islands which are insulated from one another.